1. Field of the Invention
This invention relates to a semiconductor integrated circuit incorporating a bidirectional pin.
2. Discussion of the Related Art
As is well known, external connection pins provide the terminations for a package containing an electronic semiconductor integrated circuit. They allow the package to be mounted in a quick-connect fashion on a carrier board while electric connections are made.
It is desirable in this field for the number of the external pins be minimized, both to bring down the package manufacturing costs, and to enhance the efficiency and reliability of the incorporated electronic device.
This desire clashes with the requirements of the integrated circuit designer who is called upon to consolidate and implement a number of logic functions the circuit, but has a limited number of pins for externally coupling the circuit. Another problem for the designer is that the circuit must meet the requirements of the manufacturer's packaging line. It can thus be appreciated that the current trend toward a decreased number of external pins adds to the designer's work.
For these reasons, the designer is seldom afforded a sufficient number of external pins to provide for all the functions that he or she may desire. Often the designer must accept limitations on the circuit performance and/or the potential for interfacing with external circuits.
Understandably, it would be desirable if a single pin could serve multiple functions. This has not been feasible because disturbance or interaction might occur between functions.
For example, consider FIG. 1, which shows an integrated circuit having a first digital output pin A where a voltage value may be present which corresponds to either a logic low (0-2.5 volts) or a logic high (3.5-5 volts). A second pin B is desired for digital or control input, and its state may either be, for example, an open circuit or of grounded connection. It is apparent that if pins A and B coincided, a control signal received on pin B would interfere with the proper operation of the output at pin A.
It would be desirable to provide an improved integrated circuit which overcomes the limitations of prior approaches, and which allows a smaller number of external pins to be used.